1. Field of the Invention
The present invention relates to a focus detection device used on a camera or the like that employs an appropriate TTL (Through The Lens) phase difference detection method and in particular, to a focus detection device that is ideal for detecting the focus conditions in a plurality of regions within the photo lens image surface.
2. Description of Related Art
The basic construction of a focus detection device that is used on a camera or the like and employs a TTL phase difference detection method is shown in FIG. 14. This focus detection device comprises, positioned in order on the optical axis 0 after the photo lens 1, a field of vision mask 20, a condenser lens 30, a diaphragm mask 40, an image recomposing lens 50 and photoelectric transforming elements 60, forming the focus detection optical system.
The exit pupil 10 of the focus detection optical system is in a position that is conjugate to the diaphragm mask 40 through the condenser lens 30. In the example shown in the drawing, the position of the exit pupil 10 overlaps the position of the photo lens 1. The field of vision mask 20 is positioned in the vicinity of the prearranged image composing surface (surface of film or the like).
Light rays that pass the two divided regions 101 and 102 of the exit pupil 10 form a first subject image in the vicinity of the field of vision mask 20 through the photo lens 1, the light rays being extracted through the field of vision mask 20, and the light rays from the subject image to the condenser lens 30 being restricted. The light rays that pass through the condenser lens 30 pass through openings 401 and 402 of the diaphragm mask, which restrict unnecessary light rays in the same manner and are recomposed as a second image on the element columns 601 and 602 of the photoelectric transforming element column 60 by the correction lens components 501 and 502 of the image recomposing lens 50.
In other words, two second images, which are approximately similar to the first image formed by the photo lens 1, are recomposed behind the first image on the set of photoelectric transforming element columns 601 and 602 by the condenser lens 10, the diaphragm mask 40 and the image recomposing lens 50. The focus adjustment state is detected based on the relative positions of both second images. The relative positions of the two second images on the set of photoelectric transforming columns change according to the focus adjustment state of the photo lens 1. For example, when the focus of both second images meets in front of the field of vision mask surface, the images are farther apart; and when the focus meets behind the field of vision mask surface, the images are closer together. Thus, by comparing the output of both photoelectric transforming element columns 601 and 602, the proper focus adjustment state can be detected.
With a conventional focus detection device based on the basic principles described above, the position of the exit pupil 10 is conjugate to the diaphragm mask 40 through the condenser lens 30 and is set in a fixed position through the characteristic construction of each focus detecting optical system.
In other words, since the two regions 101 and 102 of the exit pupil 10 through which light rays pass become reverse projected images of the openings 401 and 402 of the diaphragm mask 40 through the condenser lens 30, the position is fixed, and both are separate, independent regions.
The focus detection precision originates in the open angle of the centroid, or the so-called centroid open angle, of the two focus detection light rays. As the centroid open angle increases, the detection precision improves.
As indicated by FIGS. 13 (A) and (B) and by the following equation, the power corresponding to the focus adjustment amount of the focus detecting optical system, or the so-called defocus detection range .increment.Z, is determined by one side of the centroid open angle .phi., the distance L between the exit pupil 10, which is determined by default by the characteristic construction of the focus detecting optical system, and the prearranged focal surface 20 of the photo lens, and the length d of the appropriate side of the prearranged focal surface of the photo lens of the range that detects the relative position of the second image. EQU .increment.Z=(d*L)/(tan.phi.*L-d) (1)
FIG. 13 (A) shows the defocus detection range of the infinitely far side at d&gt;0; and FIG. 13 (B) shows the defocus detection range of the near side when d&lt;0.
In order to clarify the focus and improve the precision of focus detection by increasing the centroid open angle and avoiding the decrease of the defocus range on the near-side (d&lt;0), which is comparatively more difficult to secure, or in other words, of the near-side defocus detection range, it is necessary to construct a focus detecting optical system such that the exit pupil 10 is set in a position farther from the prearranged focal surface of the photo lens.
The following problems arise on the focus detection device described above.
Namely, the actual position of the exit pupil of the photo lens that will be installed is not set and varies according to the type of photo lens. There are many cases in which the diaphragm mask 40 through the condenser lens 30 and the conjugate position are different.
When the actual exit pupil position of the photo lens is markedly different, focus detection must be carried out using light rays passing through two regions that are substantially asymmetrical in relation to the optical axis of the actual exit pupil of the photo lens in the focus detecting optical system that has a focus detection region at a position outside the optical axis of the photographic surface (prearranged focal surface) and separated from the optical axis. Therefore the symmetry of the light rays is lost, a portion of the light rays is damaged according to the opening efficiency of the aperture, or in other words, vignetting occurs, and it becomes impossible to detect the focus.
In order to resolve this type of problem, methods are known, such as that clarified in Japanese Laid Open Application No. 63-284513, which reduce the diameter of the perpendicular direction in the arrangement direction of the diaphragm aperture of the set of focus detecting optical systems that have focus detection regions outside the photographic surface optical axis. These methods also decrease the area of the two divided regions of the exit pupil, which lie at conjugate positions.
However, improvement through this method is limited to focus detecting optical systems that have focus detection regions positioned in the direction of the optical axis perimeter at a position separated from the photographic surface optical axis. Improvement cannot be achieved through this method on focus detecting optical systems that have focus detection regions positioned in a radial direction of the photographic surface optical axis.
In addition, if the area of the two divided regions of the exit pupil is decreased too much, the power corresponding to the lowered light intensities pertaining to focus detection can be diminished because the amount of light from the focus detection light rays that reaches the photographic surface is decreased.
In Japanese Laid Open Application No. 1-288810 and the like, methods are provided by which the centroid space and the like of the two divided regions of the exit pupil are forced to be different by a focus detecting optical system with a focus detection region at the center portion of the photographic surface and by a focus detecting optical system that has a focus detection region at the perimeter portion.
However, since the focus detection precision depends on the centroid open angle of the two focus detection light rays in the same way as described above, when the centroid space between the two divided regions of the exit pupil is narrowed, the focus detection precision of the focus detecting optical system that has a focus detection region outside the optical axis of the photographic surface is markedly low when compared with the focus detecting optical system that has a focus detection region on the optical axis of the photographic surface. In addition, this method is restricted to focus detecting optical systems that have focus detection regions positioned in the direction of the perimeter of the optical axis at a position separated from the photographic surface optical axis and does not improve focus detecting optical systems that have focus detection regions positioned in the radial direction of the photographic surface optical axis.
The problems relating to the focus detection region positioned in a radial direction outside the optical axis are further described with reference to the conventional focus detection device shown in FIG. 6. This focus detection device, similar to the device described in FIG. 14, comprises a field of vision mask 22 positioned on the optical axis 0 behind the photo lens, a condenser lens 32, a diaphragm mask 42, an image recomposing lens 52 and a photoelectric transforming element 62. In order to make the drawing more readable, the field of vision mask 22 and the diaphragm mask 42 are shown in a position separated from the optical axis. The focus detecting optical system is formed by the field of vision mask 22, the condenser lens 32, the diaphragm mask 42, the image recomposing lens 52 and the photoelectric transforming element 62. The system has focus detection regions in a total of three places: that is, on the optical axis of the photographic surface and at positions separated from the optical axis the left and right. In each place, there is a focus detection region extending in the vertical direction, with another focus detection region perpendicular to the first and extending in the horizontal direction. Thus, on this device, it can be seen that there are essentially six focus detecting optical systems. In the drawing, Pa, Pb and Pc indicate the actual position of the exit pupil, which changes according to the photo lens.
The exit pupil of each focus detecting optical system is located in the conjugate position of the diaphragm mask 42 through the condenser lens 32. With the conventional device of FIG. 6, the "conjugate position" of each focus detecting optical system is in the same position on the optical axis 0. In this case, as shown in FIG. 7, the focus detecting optical lines that are farthest to the outside correspond to q2 and r2 and are outside the actual exit pupils Pa, Pb, and Pc. In other words, vignetting occurs. FIG. 7 also shows the optical lines that relate to the present invention for the purpose of comparison, but these will be described hereafter.
The light rays that correspond to the focus detection regions outside the optical axis are shown in the vignetting condition by the exit pupils Pa, Pb, and Pc in FIGS. 8 (A), (B), and (C). As can be seen from the drawings, vignetting only occurs for one of the optical lines of the set for the focus detection region positioned in the radial direction outside the optical axis in each case. As a result, proper focus detection cannot be carried out in relation to the focus detection regions in the radial direction outside the optical axis.